1. Field of the Invention
The present invention relates to a lid assembly for alternatively closing and providing access to a subsurface pit for servicing aircraft located beneath an aircraft servicing surface across which aircraft travel while on the ground.
2. Description of the Prior Art
At modern aircraft terminals servicing of aircraft on the ground is frequently performed using prefabricated pits which are installed at aircraft docking, fueling and loading areas beneath the surface of the tarmac across which aircraft travel during docking and departure maneuvers. The pits are typically formed of fiberglass, steel or aluminum and are constructed as enclosures with surrounding walls, and an access lid at the top of the walls. The pits are installed below the surfaces of loading and refueling aprons at aircraft terminals, remote parking locations and maintenance bases.
The purpose of the pits is to allow ground support functions to be carried out from subsurface enclosures. These ground support functions include the provision of fuel, the provision of electricity to the aircraft while it is in the docking area, the provision of air for cooling the aircraft interior, the provision of pressurized air for starting the aircraft engines, and for other aircraft support activities on the ground. The use of subsurface pits eliminates the need for mobile trucks, carts and other vehicles which are otherwise present in the loading area and which interfere with the arrival and departure of aircraft in the vicinity of a loading gate. The use of subsurface pits also allows the provision of fuel, power, cooling and pressurized air, and other supplies from a central location. The necessary fluid supplies and electrical power can be generated or stored with great efficiency at a central location, as contrasted with mobile generating or supply vehicles.
The pits located below the aircraft terminal area house valves, junction boxes, cooling air terminations and other terminal equipment that is temporarily connected to an aircraft that has been docked. Umbilical pipes and lines, otherwise housed within the pits, are withdrawn from the pits through hatches therein and are coupled to a docked aircraft to supply it with fuel, air for cooling the aircraft interior, pressurized air for starting the engines, and electrical power.
The pits are constructed with heavy lids atop hollow pit bodies. The hatches through which hoses, lines, and cords are withdrawn are seated within the lids, so that during the day-to-day operations it is normally unnecessary to open the lids. However, there are times when greater access to the pits is required. For this reason the lids are either hinged or they may be lifted and removed entirely from a lid mounting frame. This allows ground personnel to perform servicing tasks that cannot be done through the smaller hatch openings.
The lids are movable between open positions allowing access to the pits and closed positions in which the upper surfaces of the lids are flush with the surface of the docking, loading or refueling area across which aircraft travel and beneath which the pits are mounted. Because the pits are located beneath the surface upon which the aircraft travel while on the ground, aircraft servicing personnel must sometimes descend into the pit in order operate valves, free aircraft refueling lines, electrical power lines, air conditioning ducts, and to otherwise perform operations that cannot be accomplished through the relatively small hatch openings in the lids.
Some access lid assemblies are not hinged to the mounting frame. The access opening to the subsurface pit is exposed by lifting the lid vertically upwardly by means of hand grips cast into the structure of the lid. Since the lid of the pit must be heavy enough to withstand the weight of the wheels of a heavy aircraft, pit lids typically weight any where from between about 50 pounds up to 1,000 pounds or even more. Therefore, it is highly desirable for the pit lids to be hinged and for some type of lift assistance to be employed so that the pit lids can be manually raised from a seated position. Typically, conventional pit lids of this type are provided with either a counterweight system, such as that described in U.S. Pat. No. 4,467,932, or heavy duty springs to assist in raising the lid. In either case the lid is urged upwardly from a closed position seated in the mounting frame to a raised position by some mechanical assist mechanism.
One problem that exists with a gravity operated counterweight system is that the counterweights must be mounted on relatively long arms and must swing freely down into the pit as the lid is raised. This increases the space requirements within the pit to accommodate the movement of the counterweights. Also, the hoses and lines employed within the pit can become entangled with the counterweight arms or obstruct their movement.
One significant problem that exists with spring assisted pit lids is that the manual lifting force required varies considerably over the arc through which the pit lid moves from a closed to a fully opened position. This variation in lifting force required is considerable due to the heavy weight of the pit lid.
The basic problem of using springs to open a cover through a arc of 90 degrees or more is that the lifting force required to overcome the weight of the cover decreases in a nonlinear manner as the cover is raised. Specifically, the manual lifting force required, unaided by a spring or counterbalancing mechanism, decreases sinusoidally from a maximum when the lid resides in a horizontal, closed position to a minimum when the lid has been opened through an arc of 90 degrees. This is because an increasing portion of the weight of the lid bears against and is carried by the structure of the pit lid frame which transfers the force to the surrounding pavement as the lid approaches an alignment perpendicular to its horizontal, seated position. The force of an elastically deformed spring arranged to assist in opening the pit lid, on the other hand, expends its counteracting stored energy as a linear function of movement of the pit lid through the same 90 degree arc.
The spring force aiding the manual lifting force required is at a maximum when the pit lid resides in the horizontal, closed position. This maximum force must be somewhat less than the maximum lifting force required or the lid would pop open of its own accord. Current design standards require the force necessary to lift the lid from its closed position to be a force on the order of 25 pounds. Consequently, the springs must exert an opening force on the pit lid which is 25 pounds less than the weight of the pit lid when the lid is closed.
As the lid is opened, however, the moment of the spring force that aids the moment provided by the manual lifting force decreases linearly, which initially represents a much more rapid rate of decrease than the opposing moment resulting from the weight of the pit lid in rotating in through an arc. Thus, to continue lifting the lid an increasingly large lifting force must be manually applied until the lid has opened through an arc of 45 degrees. It is only at this point that the rate of decrease of the moment resulting from the gravitational force acting to close the lid exceeds the rate of decrease of the moment of the relaxing springs. As the lid approaches a vertical position, the manual lifting force required approaches zero. However, the manual lifting force that is required to move the lid through the portion of the 90 degree arc between about 10 degrees and about 60 degrees from horizontal is unacceptably large.